The handset appears slightly larger than the S3 and The Verge, which picked up the photos, says the forum claims the phone has a 1080p display, runs Android 4.2.1, has a 1.8GHz Universal5410 processor and 2GB of RAM.

Those specs, says The Verge, tally with recent rumours about the specifications of the Samsung Galaxy S4. The Universal5410, says The Verge, is "likely a codename for the Exynos Octa 5410, an eight-core processor with four A15 cores for power and four A7 cores for power efficiency".

The authenticity of the images is uncertain but they do not match pictures published by SamMobile in January, which showed a similar device but without a button on the front.

The Samsung S Pen, used with the Galaxy Note range, has an Air View feature that activates certain functions simply by holding the stylus close to the screen without touching it.

The website says it has been told by "an insider" that the Galaxy S4 will also feature a PHOLED screen, which is more energy efficient than a standard OLED screen.

SamMobile adds that it expects the Galaxy S4 to feature a 13-megapixel camera and a new photography mode called Samsung Orb that will capture 360-degree images.

Screenshots published online last week suggested that the new handset would have a 5-inch, 1080x1920 pixel display with 480 pixel-per-inch density.

Further rumours, from DigiTimes, suggest that the S4 will feature wireless charging using the Qi standard supported by Google and Nokia, rather than the A4WP standard which Samsung has backed until now.

Whatever the specifics, there is likely to be keen interest around the launch event, which takes place at midnight (UK time) on Thursday. It will be the first US launch of Samsung's flagship Galaxy phone in three years, as Samsung looks to step up competition in the crucial US mobile phone market.